The present invention relates to registration of plural image exposures formed on a photoreceptor belt by a plurality of Raster Output Scanning (ROS) systems and, more particularly, to a method and apparatus for registering the image exposures in the process direction of the belt to form registered color images in a single pass.
In a single pass, color xerographic system, a plurality of ROS stations are positioned adjacent to a photoreceptor surface and selectively energized to create successive image exposures, one for each of the three basic colors. A fourth ROS station may be added if black images are to be created as well. Thus, each image area on the photoreceptor surface must make at least three passes relative to the transverse scan line formed by a modulated laser beam generated by the ROS system. Each image must be measured to within a 0.1 mm circle or within a tolerance of .+-.0.05 mm. Each color image must be registered in both the photoreceptor process direction (skew registration) and in the direction parallel to the process direction (referred to as the fast scan or transverse registration). Registration in the transverse direction of a single pass ROS system is known in the prior art and a preferred registration technique is disclosed in copending U.S. application Ser. No. 07/635,835 filed on Jan. 8, 1991, and assigned to the same assignee as the present invention. Contents of this application are hereby incorporated by reference.
The present invention is directed towards a method and apparatus for registering the color images in the process direction only by detecting deviations in the skew of the color images being formed with respect to one another and by controlling registration errors at the lead edge of the images.
FIG. 1 shows a prior art, single pass, ROS color printing system 8 having four ROS systems, 10, 12, 14, and 16. The system 8 includes a photoreceptor belt 18, driven in the process direction, indicated by the arrow 19. The length of the belt 18 is designed to accept an integral number of spaced image areas I.sub.1 -I.sub.n represented by dashed line rectangles in FIG. 1. Upstream of each image area is a charging station (not shown) which places a predetermined electrical charge on the surface of belt 18. As each of the image areas I.sub.1 -I.sub.n reaches a transverse line of scan, represented by lines 20a-20d, the area is progressively exposed on closely spaced transverse raster lines 22, shown with exaggerated longitudinal spacing on the image area I.sub.4 in FIG. 1. Each image area I.sub.1 -I.sub.n is exposed successively by ROS systems 10, 12, 14, 16. Downstream from each exposure station, a development station (not shown) develops the latent image formed in the preceding image area. A fully developed color image is then transferred to an output sheet. Details of charge and development xerographic stations in a multiple exposure single pass system are disclosed, for example, in U.S. Pat. No. 4,660,059, whose contents are hereby incorporated by reference. The charge, development, and transfer stations are conventional in the art. Each ROS system contains its own conventional scanning components, of which only two, the laser light source and the rotating polygon, are shown. The particular system 10 has a gas, or preferably, laser diode 10a, whose output is modulated by signals from control circuit 30 and optically processed to impinge on the facets of rotating polygon 10b. Each facet reflects the modulated incident laser beam as a scan line, which is focused at the photoreceptor surface. Control circuit 30 contains the circuit and logic modules which respond to input video data signals and other control and timing signals to operate the photoreceptor drive synchronously with the image exposure and to control the rotation of the polygon 10b by a motor (not shown). The other ROS systems 12, 14, 16, have their own associated laser diodes 12a, 14a, 16a, and polygons 12b, 14 b, 16b, respectively. In the system of FIG. 1, transverse alignment of each successive image exposure is obtained by providing horseshoe shaped sensors 36a, 36b, 36c, 36d, which cooperate with optical targets T1, T2, T3, T4, respectively, formed in the belt surface. Further details regarding transverse alignment registration are described in the aforementioned application Ser. No. 07/635,835. However, for this prior art system, a skew or process alignment must also be accomplished to ensure complete registration of the multiple image exposures.
One of the main causes of skew error is due to belt conicity in the photoreceptor belt. Belt conicity is created when the two ends of the photoreceptor sheet are welded together to form the belt, causing the two belt edges to be of slightly different lengths. Another factor is the "set" that the belt takes over the life of the belt due to lateral deviation in tension roll or steering roll forces. A third source of potential belt conicity is the machine warm-up difference in temperature gradients from machine front-to-back causing lateral distortion. A still further potential source of conicity is movement of the photoreceptor module during, for example, a jam clearance. Any of these might create a situation, referring to FIG. 1, wherein the leading edges of images I.sub.1, I.sub.2, I.sub.3, I.sub.4 would rotate as they translate from one position to the next. If images I.sub.2, I.sub.3, I.sub.4 are to be perfectly registered with image I.sub.1, the leading edges must not be parallel to each other but must accommodate the rotation induced by the conicity of the belt. Since the degree and direction of the conicity of the belt varies from belt to belt, each ROS system must be individually aligned to correct for the initial misregistration.
According to the present invention, a method and apparatus is provided for aligning ROS units in a single pass printing system, so that each ROS is aligned along the process or X-axis, so as to compensate for belt conicity and other registration errors. After this alignment, the images formed by each ROS will be in proper registration within the prescribed tolerances. The rotational alignment in the X-direction, also referred to as a skew alignment, is made by sensing exposure lines formed by each ROS through apertures which extend transversely and are at opposite ends of the photoreceptor belt.
The ends of each scan line of each ROS are sensed simultaneously, and, if the detected signals are not coincident in time, an error signal is generated and applied to a precision, linear actuator, such as a stepper motor, which, in turn, transfers the linear motion to one of the optical components of the ROS system. In a preferred embodiment, a folding mirror is selected to be movable by the actuator, so as to change the angle of the projected scan line to correct for a detected skew error. This skew registration is accomplished for the first ROS system and then is repeated for each of the ROS systems until all four leading edge exposures, 20a, 20b, 20c, 20d, are sensed through the two skew apertures in coincidence, ensuring process registration of the associated color images. More particularly, the present invention relates to an imaging system for forming multiple image exposure frames on a photoconductive member during a single pass including:
a photoreceptor belt adapted to accommodate the formation of an integral number of image exposure frames, said belt having a first and second alignment aperture on opposite sides of the belt width and outside of the exposure frame,
a plurality of Raster Output Scanners (ROS) units, each ROS unit associated with the formation of one of said image exposure frames, each ROS unit forming a plurality of projected scan lines in a fast scan (traverse) direction across the belt width, said scan lines beginning and ending at points outside of the image exposure frame,
first and second detecting means associated with each of said ROS units, said detecting means adapted to sense the projected scan lines when they become visible through said alignment apertures and to generate position signals indicative thereof, and
means for rotating said scan line to correct for process registration errors until the detected position signals from said first and second detecting means are concurrent.